Reference may be made to the following U.S. patents of interest: U.S. Pat. Nos. 3,470,943; 4,138,851; 4,161,657; 4,189,924.
Various power generating systems utilizing low temperature differences existing in natural conditions in the United States and other countries, such as geothermal, solar, solar ponds, ocean thermal energy, etc. are presently under consideration or in some cases pilot plants have been constructed. In an ocean thermal energy conversion system described in the aforementioned U.S. Pat. No. 4,189,924, warm ocean water (at or near the surface) is vaporized and the resulting steam is used to drive the turbine/electrical generator. The exhaust steam is then condensed using relatively cold ocean water with the mixture then being returned to the ocean.
In proposed improved ocean thermal energy conversion power generating systems, the use of ammonia, propane or freon or other working fluids having a relatively lower boiling point than water have been proposed. As an example, it has been proposed to use ammonia as the working fluid in an electrical power plant based on a closed Rankine cycle utilizing the ocean temperature difference inherently available in the vicinity of Florida, Hawaii, Puerto Rico, etc. The working fluid, such as ammonia, is directed to an evaporator, where it is vaporized by heat supplied from warm ocean water. The saturated ammonia vapor is then fed to a turbine/electrical generator and thereafter to a condensor where cold ocean water condenses the ammonia which is then pumped back to the evaporator to complete the closed loop cycle.
However, if a small percentage of water exists in the loop, (either from a leakage or due to the initial charge of working fluid containing water), the water will concentrate in the evaporator since it is less volatile than the working fluid. The presence of water will increase the saturation temperature, relative to that of anhydrous ammonia, and in addition will cause a reduction in the heat transfer performance of the evaporator. As a result, the efficiency of the evaporator is decreased significantly which results in a decrease in the net power output of the system.
The desired separation of water from the working fluid, such as ammonia, is a very challenging engineering task since a large amount of ammonia has to be treated to remove only a few percent of water. Thus, while known available techniques for separating water from ammonia or similar fluids, such as distillation, membrane separation, or adsorption, have been proposed and could be utilized, they introduce a substantial amount of apparatus to the system, thereby making the system more complicated, subject to more frequent maintenance, and increase the overall system cost.